Many synchronous machines, such as motors designed to be started with a thyristor soft-starter, have or use a brushless direct current (DC) exciter. The exciter is used to help overcome the inertia associated with accelerating the rotor from a resting position to full speed.
In various situations, it is desirable to operate a synchronous motor with a brushless DC exciter with a medium voltage, variable-frequency drive (VFD) motor controller such as those described in U.S. Pat. No. 5,625,545 to Hammond. A medium voltage VFD motor controller provides a required initial current for start up of a synchronous motor with a brushless DC exciter while avoiding a voltage drop in the utility supply voltage. However, for operation without a requirement of a speed sensor, a synchronous motor with a brushless DC exciter should be synchronized before significant load is applied. Otherwise, the machine may slip a pole and cause the flux to decrease rapidly, sometimes causing a loss of speed control. Accordingly, using a VFD without a speed sensor to start a synchronous motor having a brushless DC exciter can be difficult.
One option to overcome this difficulty has been to replace the DC exciter with an alternating current (AC) exciter so that excitation can be applied at standstill. However, in many situations this course of action can be mechanically difficult, as it requires replacement of a part in often hard-to-access locations.
The disclosure contained herein describes attempts to solve one or more of the problems listed above.